A Study Of Die Bonding On Gold-plated Substrate Using Nanosilver Paste

As a new type of lead-free die attachment material,nanosilver paste has excellent high temperature stability,electrical and thermal conductivity,and meets the requirements of high temperature and high density packaging in high power and wide bandgap semiconductor devices.However,gold-plated substrate as an important packaging material in the field of electronics and aerospace,the study on its interconnection performance with nanosilver paste still has some problems to be solved.Therefore,it is of great significance to study the interconnect mechanism and sintering process of low temperature sintered nanosilver paste with gold-plated substrate.Firstly,in this paper,the quality of the sintered silver joints of electrolytic Au and ENIG was compared.It is found that the fracture pattern,SEM morphology of fracture surface and cross section was different between the sintered Ag joint with different Au finishes.Among them,the electrolytic Au sintered Ag joint has the best quality,dieshear strength arrived 20 MPa,but there is an obvious defect of delamination at ENIG sintered Ag joint,which reduces the quality of sintered Au joint of ENIG and directly leads to low die-shear strength about 7.5 MPa.In reaction to the phenomenon,in this paper,the interconnection mechanism of nanosliver paste bond with ENIG substrate was made the following hypothesis: the sintering process of nanosilver paste and goldplated substrates,the diffusion rate of Ag diffuse into ENIG is higher than that of Ag diffuse in to electrolytic Au,the sintered Ag could also form solution with Au finishes during the sintering of nanosilver paste at high temperature.Large number Ag atoms of the nanosliver paste adjacent to the ENIG are going to be diffused to the Au finishes in the sintering process,the left Ag atoms of the nanosilver paste in the vicinity of the interface are not sufficient to be densified as a robust sintered Ag joint because the atomic inter-diffusivity of Ag to Ag is much less than that of Ag to Au.Eventually voids or even delamination could be present at the sintered Ag in the vincinity of the interface after sintering.Then,in order to verify the validity of the above mechanism,the microstructure of electrolytic Au and ENIG was tested by electron backscatter diffraction(EBSD).The results show that the number of small grains and large angle grain boundary of ENIG substrate is larger than that of electrolytic Au substrate,at same time there are(111)texture at the ENIG substrate.All these microstructures show that the diffusion driving force of Ag diffuse into ENIG is lower than that of Ag diffuse into electrolytic Au,therefore,the diffusion rate of Ag atoms in ENIG is higher than that of Ag atoms in electrolytic Au.This result is consistent with the above bonding mechanism.Finally,based on the interconnection mechanism of nanosilver paste bond with the ENIG,the sintering process was modified.Reducing sintering temperature can improve the quality of sintered Ag joint of ENIG.Preheating process and short time sintering at low temperature also improved the quality of sintered Ag joint of ENIG.This paper also attempts to further decrease the thickness of the Au layer,so as to further ease the Ag and Au atoms form infinite solid solution,inhibition of Ag-Au interface Ag atoms appear large area lack.The study found that the die shear strength of 80 nm immersion gold layer of ENIG than that of 300 nm immersion gold layer of ENIG is higher.